One of the most important emerging behaviors in biology is swarm formation. Many species of bacteria swarm, including species found in diverse soil and water environments such as Bacillus subtilis, Serratia liquefaciens, Proteus mirabilis, Pseudomonas aeruginosa, and Myxococcus xanthus. These and other swarming bacteria span the gamut of utility and range from innocuous carbon-cycle organisms to harmful pathogens. Swarming is observed in cells that are propelled by rotating flagella, by the secretion of slime, and by retracting type IV pili. Bacteria move on substrates and change their local environment in a way that improves cellular physical interaction and optimizes swarming. M. xanthus, a rod-shaped, Gram negative myxobacterium is able to glide on surfaces using two genetically independent yet cooperative A and S motility engines. These bacteria produce slime and move on slime tracks produced by other members of the colony. Remarkably, these bacteria also regularly reverse their gliding directions. The main goal of this proposal is to combine simulations using new three-dimensional multiscale modeling environment and specifically designed experiments to study basic coordination events of M. xanthus swarming, which is essential to understanding how millions of bacteria function in real environments. Specifically, we will study the role of flexibility of cells, viscosity of extracellular polysaccharide, slime adhesivity and directional reversals in resolving collisions, increasing alignment and optimizing swarming rate during swarming of mutant strains (A+S-) and ( A-S+) and wild type (A+S+) of M. xanthus. Predictive simulations will yield new biological hypotheses about M. Xanthus swarming because of the ability to conduct "experiments" in silico that are yet difficult (or impossible) to perform physically. A key aspect of this work will be to compare predictions obtained in silico with experimental observations. Study of the M. xanthus social interactions will provide an opportunity to gain fundamental insight into the biological response to how organisms discern, process, and respond to the chemical, physical, and biological cues present in their local environment.